Conventionally, as one of practical methods for executing three-dimensional measurement of a measurable object like an automotive body, there is such a method to compute dimensions of a measurable body via triangulation based on image data picked up by a plurality of two-dimensional photosensitive elements like CCD cameras for example.
According to this conventional method, as shown in FIGS. 5 and 6, when light beam from a light-point P, formed at a predetermined position on a measurable object A, is incident upon a light-receptive solid sensor 3 through a lens 2 of each CCD camera 1, among a number of picture elements 4 conjunctionally composing the solid sensor 3, a computer unit detects a specific position from the center of the solid sensor 3 corresponding to the center of the output from a group of picture elements 4 respectively emitting image output in response to the incident light beam, and then, based on the positional data, the computer an computes angle of the light point P emitted from respective CCD cameras 1. The reference character "L" shown in FIG. 6 designates the light axis of each CCD camera 1.
Next, the computer unit computes the position of the light point P based on the angle data transmitted from a plurality of CCD cameras 1.
In this way, any conventional system determines three-dimensional dimensions of a measurable object A by sequentially computing data collected from a number of light points P on the measurable object A.
For example, a laser scanner may be used for composing practical means for forming a number of light points P on a measurable object A by sequentially irradiating the object A with a laser spot beam.
However, in order to execute a three-dimensional measuring process based on the above method, it is essential that an absolute positional relationship of an individual CCD camera 1 against the measurable object A be detected prior to execution of an actual measuring operation.
To achieve this, prior to the execution of a three-dimensional measurement by means of a plurality of CCD cameras 1, initially, a plurality of measuring-point members 10 available for an optical measurement as the basis of the measurement, are installed on the measurable object A or at plural positions in the periphery of the measurable object A. Next, the computer unit computes absolute positions of the respective CCD cameras 1 based on the positions of the respective measuring-point members 10 seen from the respective CCD cameras 1 and also based on the distance between the previously known two points on or in the periphery of the measurable object A.
As shown in FIG. 4, normally, the conventional system uses those measuring-point members 10 which respectively use an LED light source and incorporate a cap lens 13 in the center of the front surface of a cylindrical system 11, where the cap lens 13 protects an LED light emitter 12 and evenly irradiates a front domain of the light emitter 12 by an emitting light beam.
As mentioned above, each of those conventional measuring-point members 10 for an optical measurement incorporates an LED light emitter 12 having a substantial area, and in addition, a cap lens 13 for protecting the LED light emitter 12 is installed around an external periphery of the light emitter 12. Because of this structure, when each of those CCD cameras 1 picks up an image of the individual measuring-point members 10, the LED light emitter 12 itself is identified as a light ball having a substantial area, and yet, whenever varying an angle to the pick up image of respective measuring-point members 10, due to frictional effect of the cap lens 13 disposed in front of the LED light emitter 12, the position of the most luminous domain inside of the light ball is variably seen.
Therefore, whenever optically measuring three-dimensional dimensions of a measurable object A based on high precision by utilizing those measuring-point members 10, both the magnitude and apparent shift of the most luminous domain in the light ball, which adversely affects precision of the measured effect to be lowered is a critical problem to solve.